Cycloidal propeller having low frictional drag losses of the rotor



Aug. 30, 1960 D. L. STEPHENS 2,950,764

CYCLOIDAL PROPELLER HAVING LOW FRICTIONAL. DRAG LOSSES OF THE ROTOR Filed Jan. 25, 1958 3 Sheets-Sheet l 8'. E w s 8 l x; g n r I I l Q g p 4 o o H (O O 9 (SI! mg f W/lllllllllllllllllll //I N A -INVENTOR N DONALD STEPHENS ATTORNEY 1960 D. L. STEPHENS 2,

CYCLOIDAL PROPELLER HAVING LOW FRICTIONAL DRAG LOSSES OF THE ROTOR Filed Jan. 23, 1958 3 Sheets-Sheet 2 INVENTOR DONALD STEPHENS ATTORNEY 1960 L. STEPHENS 2, 50,764

CYCLOIDAL P EL ING CTIONAL DRAG LOS u THE 0 Filed Jan. 23, 1958 3 Sheets-Sheet 3 INVENTOR DONALD STEPHENS g BY ATTORNEY United States Patent ice g Aug CYCLOIDAL PROPELLER HAVING LOW ERIC-- TIONAL DRAG LOSSES OF THE ROTOR Donald L. Stephens, Bellevue, Wash., assignor to Pacific Car and Foundry Company, Renton, Wash.

Filed Jan. 23, 1958, Ser. No. 710,799

2 Claims. or. 170-147 My invention relates to cycloidal propellers of improved efliciency and pertains particularly to a means for streamlining the rotor housing so as to reduce that power lost in fluid friction between the rotor and its surrounding recess in the ships hull.

In propellers of the type improved by my invention, a drum-like carrier rotor is rotatably mounted in a ships hull with one end surface set flush with an unobstructed horizontal region on the underwater hull. This rotor, driven by the ships power plant, carries a plurality of fiat, plate-like impeller blades extending generally downward and rotatably mounted from equi-spaced positions near the periphery. As the ship is moved through the water, the rotary motion of the blades superposed upon the general unidirectional translation of the ships hull, and thus of the rotor as a whole, produces a generally cycloidal pattern of motion for each blade when viewed from fixed coordinates. In view of this motional pattern, propellers of this type are generally designated as cycloidal, and this designation should be understood to apply in the following description.

Cycloidal propellers provide a flexibility of control for a ships helmsman -far surpassing that available with the more commonly used helical screw propeller combined with a rudder. In addition to controllable pitch of the driving blades, the full thrust available from the propeller may be directed horizontally at any desired angle with respect to the longitudinal axis of the ship. These important advantages are attended, however, by some problems which are peculiar to cycloidal propellers. For in order to produce an orbital motion of blades pivoted about axes generally parallel to the main axis of rotation, the supporting platform or hub must be of substantial lateral dimension. The blades, being attached at only one end, must be cantilevered in bearings which are separated by appreciable distance so as to counter the bending moment produced by blade loading while maintaining reasonable bearing loads. Separation of the blade bearings imposes a requirement of minimum depth on the rotor drum, thereby fixing an appreciable vertical dimension of the blade carrying rotor. Thus, compared to the axial hub of a helical screw propeller, 'the rotor of the cycloidal propeller is quite large. Since it must be rotated while immersed in the water, a more substantial frictional drag results from the fluid friction between the outer surface of the rotor and the surrounding water or bounding well. Thus it is desirable that the shape of the non-propelling parts of the rotor be as hydrodynamioally smooth and streamlined as possible so as to reduce power losses due to turbulence.

In cycloidal propellers heretofore proposed, little attention has been devoted to this problem of'rotor frictional losses. Rotors, in addition to their necessary large size, become hydrodynamioally unclean as a consequence of substantial protuberances from the upper surface of the rotor. The largest obstructions are generally formed by pot-shaped housings for the inboard propeller-blade bearings which, to obtain additional leverage, are set out from the upper surface of the rotor drum-housing. Water entrapped between the rotor assembly and the encasing well in the ships hull is impelled to rotate in step with these positive protuberances from the rotor housing. Movement of this water along the inner surfaces of the encasing well is attended by local turbulence requiring, in the net, extra power which adds nothing to the propulsive thrust of the propeller.

I have discovered that the frictional drag losses in cycloidal propellers can be substantially reduced if the water, which is rotated essentially in step with the rotor top by impelling protuberances, is prevented from reaching the side walls of the bounding well. To accomplish this I propose to extend the cylindrical walls of my rotor housing upward into close proximity with the top of the bounding well. When impelled only by skin frictions on my rim-like extension, the tangential fluid velocity at the surface of the bounding well is, on the average, only half that of the rotor itself. Asa result, the frictional losses, which vary as the square of this tangential velocity, will be only one quarter that for the case in which water is impelled in step with the rotor, for the area of the Well opposite my rim-like extension.

It is therefore one object of my invention to reduce the frictional drag attending rotation of a blade carrying rotor in an immersed well.

Another object of the invention is to provide increased structural rigidity and strength to the blade carrying rotor.

improved efficiency and increased rigidity with a structural addition which does not interfere with the normal assembly and serving of the propeller rotor.

These and other objects of my invention will become apparent from the following description when taken together with the accompanying drawings in which:

Fig. 1 is a sectional elevation through a vertical central axis of a cycloidal propeller which has been modified to reduce rotor losses;

Fig. 2 is a sectional plan view taken along the line 2-2 of Fig. 1 and showing the top of the rotor housing;

Fig. 3 is an enlarged sectional view along line 33 of Fig. 2 showing my rim-like extension in more detail;

Fig. 4 shows a vertical plot of the tangential fluid velocity adjacent to the stationary-housing wall for a unshielded rotor housing; and

Fig. 5 shows a diagram, similar to that of Fig. 4, for a rotor housing modified in accordance with the teaching of my invention providing a smooth barrier rim between the rotor top and the stationary housing.

Referring now in detail to the drawings in which like reference characters denote like parts, the scheme of operation of propellers of the cycloidal type can be illustrated from Fig. l. A detailed description of a similar propeller is given in a copending application Serial No. 710,798, filed January 23, 1958, so that only components essential to the rotation of the drum will be outlined here by way of illustration. A portion of the hull plating of a ship at the position where the propeller is to be located 1 is provided with a circular recess having cylindrical side walls 2. and top cover plate 3. Closely fitting the interior of this recess, a rotor, defined generally by cylindrical side walls 4 and top cover plate 5, is nested with bottom cover plates 6 and 7 set flush so as to continue the smooth contour of the outer hull 1. Rotatable support for the rotor is provided by its attachment to the flange shaped lower extremity of hub '8 which in turn is journaled in low-friction main bearings 9 at the top and 10 at the bottom. Central cylindrical bosses 12 and 13 attached to the fixed structure provide seats for bearings 9 and 10; When the rotor is stationary, it is immersed in whatever buoyant liquid is supporting the ship. Liquid which fills V 3 the well 2 and 3 is prevented from leaking into the ships hull by seals 11 operative between the lower end of hub 8 and cylindrical central extension 13 of top cover plate 3.

In the usual application, power to drive the vertically disposedmain rotor shaft Swill come from a horizontally disposed output shaft '14 of the ships power plant.- 1 A right angle drive and speed reducer is provided by the use of a beveled pinion gear 15 attached to'ou tpu t'shaft 14 'and enmeshed with a larger beveled ring gear 16 which is centered on shaft 8 by supporting hub 17 ration is obtained by mounting of the upper bearing in a cap 22 which rises above the outer surface of top cover plate '5. Inthis way the height of the rotor box can be held to values compatible with the vertical space requirements of the blade. orientation control system, while still maintaining reasonable bearing loads. Protrusion of the bearing caps 22 above the upper surface of the rotor isnot Without disadvantages, however, as these act as impellers for the entrapped water and whirl it at a higher rotational velocity than would be the case for an unobstructed top cover disk. It is this increased rotational velocity which,

' inproducing an increased tangential velocity gradient ad.

tional dragon the rotor. q V

-While it would conceivably .be possible to completely encase the upper bearing holders 22 'within the confines of a smooth drum-shaped ro-tor, must of the maximum possible reduction in friction can be realized by extending only the outer wall 4 with a vertical rim member 23 rising into close, proximity of the top cover plate 3. Smoothing of the top of the rotor would be largely ineifective since when; the rotor is turning at its designed rnnning speed, centrifugal forceacting on the rotating entrapped water jacentthe inner wall of ell 2, results in increased fricforcesit outward to the peripheral regions of the rotor; V

The inner regions of the rotor top operate in a pocket of airwhich is drawn in through a'vent 21-14 1 and thus con tributesnothing to turbulence. of the liquid and. consequent power'loss. V V w 7 V i i a A qualitative comparison of the velocity patterns exist ing between the old and new'typerotors and the .fixed housing is helpful to an understanding of the advantages accruing from the present invention. In order to' determine the'energy or horsepower loss, it is only necessary to consider the friction forces. acting on the inner Wall of well 2 as conservation of momentum dictates thatfricclearances, the Reynolds number is such that the flow is turbulent. In such a case, the velocity of fluid in the gap When my rim-like extension is added, the velocity in the gap (Fig. 5 cannot be as high as the tangential velocity of the rotor, or there would beno velocity gradient between the water and the rotor rim by which frictional force is developed and the water is moved. By the same reasoning, the water cannot remain stationary. If the surfaces of rotor extension 23 and'well wall'2 are equally rough (or smooth) symmetry requires that themid-gap velocity be just one-half the peripheral velocity of the rotor. A halving of the velocity adjacent to the ;well wall would mean a reduction of friction-force reaction here to onequarter that for the case of an unshielded rotor top. The power loss, which is proportional to' the product of rotor speed and friction force", will thereby be reduced to only one-quarter of that lost with anunshielded rotor top.

Addition of my circumferential rim shield 23 is further beneficial in that it increases the rigidity of the rotor housing structure. Maximum stiffness of this structure is desirable to reduce distortion around the :blade shank bearings when the propeller blades are undermaximum' load. This 'is particularly helpful in localizing damage resulting from blade impact with underwater obstructions.

By confining the rotor shielding 'to a circumferential rim, assembly and .disassembly of the propeller is not I further encumbered. Thus the rotor caps 22 can be redru'm c onnectedito the drive shaft immersed in a fluid and having cylindrical sidewalls, an upperface, and a lower face, a plurality of propeller blades extending from the lower face of the rotor drum an axle shaft attached to each propeller'blade, a roiatahlegmounting for each axle shaft in the drum including an upper bearing for each axle shaft projecting external to theupper rotor face, a cover pot for each upper bearing, avvellencasing said rotor having sidewalls surrounding the rotor cylindrical side Walls and a top bounding the upper rotor face and cover pots, a vent to the atmosphere taken from the top of said well a extending from the cylindrical walls of the 1 tional'resistance. on the rotor shall be equal to this. At 1 commonly used rotor peripheral velocities and radial will be nearly constant except in regions very close tothe my circumferential rim (Fig. 4-) the water adjacent the i well wall is impelled by the necessary protuberances 22 on the top of the rotor housing to rotate: bodily with the rotor, and thus obtain the tangential velocity of the rotor periphery. r, 5

rotor into close proximity of the top of said well, said rim serving to isolate any liquid carried along with the rotor top from tangential slippage against the side walls of the well, the rotor friction beingthereby reduced. I V

2. In a cycloidal propeller, a drive shaft,. a rotor drum attached to the drive shaft ha ving substantially cylindrical sidewalls, an upper face anda lower face, .a plurality of propeller blades projecting from. the lower face of the rotor drum, a corresponding plurality of positive protuberances. projecting from the upper 'face of the rotor drum, a stationary Well encasing the .side wallsand upper face of the rotor drum, a vent to the atrnsphere from the sta arrw l ai me rer ens in q h i 'drical sidewalls of the well above the level of the positive protuberances projecting therefrom,'said rim serviug to prevent .anyfluid carried along with the rotor top from centrifugal. impingement against the stationary 'well and thereby reducing fluid friction losses. 7

References Cited in theme of thispatenta fUNI TED sTArEs PATENTSIITI h 2.153. 0 f z Ji ,5

P T I. E TE .E 7 1,124,830 France v Oct. 18,1956 

